1. Field of the Invention
The present invention relates to holography and, more particularly, to an apparatus for making a reflection type Holographic Optical Element (HOE).
2. Description of the Related Art
An HOE is a hologram that functions as an optical element, such as a lens, a diffuser, a filter, or a mirror or a hilogram that performs a combination of these functions. The HOE has an advantage in that an optical system can be a small or simple structure. However, the HOE is so difficult to manufacture that it is used only for special purpose optical display elements such as a filter for spectral application, a Head Up Display (HUD), and a Helmet Mounted Display (HMD).
In manufacturing a reflection type HOE, interference patterns are generated that include a main interference pattern and a spurious or parasitic interference pattern. The main interference pattern is generated as an input beam, reflected by a mirror or an object, interferes with another input beam directly transferred from a laser. The spurious or parasitic interference pattern has been found to be generated as a beam, which was reflected from an object and secondly by the surface of a holographic photoplate making contact with an air, which interferes with another input beam or the beam that reflected from a mirror. One of the problems associated with manufacturing a reflection type HOE is that this interference pattern degrades the HOE by increasing the bandwidth and decreasing the diffraction efficiency such that additional flares appear around the reconstructed image. When the HOE is applied particularly to an display device such as a HUD and HMD for an aircraft, the quality degradation of the HOE requires that the output beam of the projector should be strong enough to generate a bright image. In this case, more power is consumed and overall device weight is increased, thereby decreasing the efficiency of the device. However, an aircraft should be small and efficient, since the power supply and weight are limited. Further, a filter used for a spectrum analysis should pass only light of specific wavelengths and reflect light of the other wavelengths, which requires that the diffraction efficiency should be 100 percent to reflect light of such wavelength.
Conventional approaches in attempting to make a high quality reflection type HOE will be explained. The first approach photographs a hologram after positioning a master glass block that has the same refractive index as a substrate glass of a photoplate, and a thickness larger than the coherent length of a laser over a photoplate (as disclosed in SPIE 1993, Vol. 1988, p. 143). The second approach records through a cover glass plate connected to several piezo elements over a photoplate (as disclosed in U.S. Pat. No. 4,458,977 issued on Jul. 10, 1984). The third approach compensates the influence of a plate reflective beam by using two beams (as disclosed in U.S. Pat. No. 4,458,978 issued on Jul. 10, 1984). The first and second approaches prevent interference by dephasing the beam reflected from a plate. If a master glass block is used, the type of master glass should be made to correspond to the type of HOE, and aberrations such as an astigmatism is generated as the thickness is increased, degrading the quality of a HOE. Also, the manufacturable size of a HOE is limited, since the glass block becomes difficult to manipulate as the size of a glass block is increased according to the size of a HOE. If piezo elements are alternatively used, since a cover glass corresponding to the type of photoplate is vibrated by the piezo elements over the photoplate, the vibrating piezo elements could vary the gap between a photoplate and an object mirror via a refractive index matching fluid. Further, as the size of the cover glass increases, the vibration by the vibrating piezo element becomes more difficult to generate, thus the manufacturable size of the HOE is limited. The approach of using two beams is difficult to implement, since two beams must be controlled by matching independently.